When discharge lamps are operated, a plasma discharge arc is produced between the electrodes and this emits electromagnetic radiation. The electrodes consist mostly of tungsten, because tungsten is a tough material which only melts at very high temperatures. In particular in the case of short-arc lamps, in which the electrodes are subject to great stresses, high electrode temperatures arise. As a consequence, evaporation of electrode material occurs at the tips of the electrodes, and this deposits on the inner side of the lamp bulb, resulting in blackening of the bulb. This blackening inevitably has the effect of an unwanted reduction in the strength of the radiation during the burning time.
Especially in the case of lithographic patterning of semiconductors, a reduction in the radiation strength results in a lengthening of production times due to the longer exposure times, and in extreme cases can necessitate a premature lamp replacement.
The vapor pressure of any material increases exponentially with rising temperature, so that it is possible by reducing the electrode tip temperatures to efficiently reduce the vapor pressure, and as a consequence the material erosion at the electrode tips and, thereby ultimately also reduce the blackening of the bulb. Such a reduction in temperature can be achieved by an emission-raising coating on the electrode.
From WO 00/08672, an electrode for a high-pressure discharge lamp is known which uses a dendritic layer of rhenium or other high melting-point metals. The term dendritic layer is to be understood as a nanostructure which is formed by numerous needle-shaped growths on the otherwise smooth surface. The surface of such a dendritic layer appears dark grey to black, and achieves an emission coefficient of over 0.8. The operating temperatures on an anode plateau can thereby be reduced by up to 500 K compared to uncoated anodes. A disadvantage of dendritic layers of this type is the high expense of manufacture and the associated high costs. The application of dendritic coatings by means of CVD or PVD techniques is very costly. Furthermore, burning time tests on lamps subject to great stresses with such anode coatings have shown that even the dendritic needle structures lose their initial form over the course of the service life, and thus the anode loses its original good emissivity.